The mole

The mole (mol) is the amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. It is the SI unit for the amount of substance. Malay: mol · Chinese: 摩尔.

Because chemical reactions happen between unimaginably large numbers of particles, the mole gives chemists a convenient counting unit, in the same way that a dozen means twelve. One mole always contains the same number of particles no matter what the substance is, and those particles may be atoms, molecules or ions, so you must always state what you are counting. The mole lets chemists move between the world of single particles, which are far too small to weigh, and the masses and volumes they can actually measure in the laboratory.

Example. One mole of any substance contains 6.02 x 10²³ mol⁻¹ particles. So one mole of water contains that many water molecules, and one mole of sodium contains that many sodium atoms; the count is identical even though the masses differ. This is why a mole of a heavy substance weighs more than a mole of a light one, while both hold the same number of particles.

Confusion to avoid. Do not confuse the mole with the molecule. A molecule is a single particle, while a mole is a huge, fixed number of particles, so the two words are not interchangeable. Take care too to say “one mole of oxygen molecules” or “one mole of oxygen atoms” clearly, because a mole of O₂ molecules contains two moles of oxygen atoms.

In this chapter the mole is the hub that connects mass, number of particles and the volume of a gas, because each of these can be converted to and from an amount in moles. Mastering what the mole means, and quoting the carbon-12 comparison in the definition, is the foundation for every calculation across SPM Chemistry.